Electronic devices are conventionally produced on a “wafer” of silicon. Nowadays, such wafers can have diameters up to 300 mm, bearing thousands of individual devices in a rectilinear array. After fabrication of the devices on the wafer, each device requires testing. The wafer must then be accurately cut up into sections bearing individual devices, which can then be processed further to make electronic chips and the like. It is important to align the wafer precisely with respect to the testing apparatus, so that the testing apparatus may conveniently and automatically locate and address each individual device. It is equally important, given how closely devices are packed on to wafers, that the wafer is precisely aligned before being cut up into individual devices.
Wafers are conventionally handled and transported on “sawframes”. A sawframe has a generally annular shape, cut from a sheet of stainless steel, aluminium or plastics material. A thin support film of plastics material extends across the circular central aperture of the sawframe, and the wafer itself is mounted to this support film. The outer periphery of the sawframe is not completely circular, but has four straight edges or “flats”, usually four, arranged as sections of the sides of a concentric square. These may be referred to as the major flats. The outer periphery of the sawframe is also provided with a plurality of minor flats and other indentations (which may be referred to in general as “notches”) the exact form and arrangement of which may vary between manufacturers. These notches or minor flats are each precisely aligned with respect to one of the major flats.
After fabrication, a silicon wafer is mounted to the support film, with the rectilinear array of devices on the wafer being aligned with the major flats of the sawframe with a very high degree of accuracy. The wafer may then be transported safely, with only the sawframe needing to be handled.
The wafers supported on sawframes are generally transported in cassettes, each of which may typically hold up to twenty five sawframes. A cassette comprises an open-ended frame with a plurality of horizontally opposed pairs of horizontal flanges mounted internally to opposing walls of the frame. A single sawframe may be supported between each such pair of flanges, and may be inserted into and removed from the cassette horizontally, through an open end of the frame. The cassettes are generally dimensioned such that two opposite major flats of each sawframe may be approximately aligned with the opposing walls of the frame. However, the sawframes are not a tight fit, in order to facilitate insertion and removal, and so may move around to an extent within the cassette during transport. As a result, the exact alignment of the sawframes relative to the cassette can vary by several degrees on either side of a centre line of the cassette.
Silicon wafers are usually handled in “clean rooms”, so robotic devices are preferred for most handling operations. The sawframes are usually removed from cassettes with a robotically-controlled arm ending in a parallel pair of flat prongs. The prongs may be inserted between a pair of flanges of the cassette, then brought up from below the sawframe resting on that pair of flanges, to lift the sawframe from the flanges for removal. A solenoid-operated clamp, is provided adjacent to prongs to bear down on the top of the sawframe, preventing it moving relative to the prongs during handling.
Once transferred to the testing and cutting apparatus, the sawframe and the wafer thereon may be precisely aligned using microscopic imaging and very accurately controllable (x,y) stages. However, such alignment methods are only useful in practice when a sawframe and wafer are presented which are already close to the optimum alignment, It is normal, therefore, to remove a roughly-aligned sawframe from a cassette and to transfer it initially to an intermediate alignment apparatus. This apparatus is provided with two parallel steel jaws, which are closed into contact with opposite major flats of the sawframe. This brings the sawframe and its wafer into sufficiently accurate alignment that, on subsequent transfer to the testing and cutting apparatus, they may be precisely aligned using the (x,y) stages thereof, as described above. The movement of the robotic arm is sufficiently accurate to maintain the alignment of a sawframe once it has picked the sawframe up.
It is inconvenient and expensive to provide such an extra piece of alignment apparatus, and the added alignment steps slow down the processing of the wafers. It would be preferable if the robotic arm could improve the alignment of a sawframe while handling it, eliminating the need for an intermediate alignment apparatus and processing step.
A further problem with the wafer handling system in its current form is the operation of the clamp to hold the sawframe in position on the robotic arm. The solenoid-operated clamps currently employed can be jerky in operation. They also provide no feedback mechanism to indicate whether a sawframe has been securely gripped. When picking up a badly-aligned sawframe, it is possible that the existing clamps will miss the sawframe or only grasp it partially, allowing it to move further out of alignment as it is transferred, or even leaving the sawframe in the cassette as the robotic arm withdraws.
There is hence a need for a more controllable means of holding sawframes in place, as well as a need for a means of indicating whether a sawframe is securely held before it is moved.
It is therefore one object of the present invention to provide a device for handling sawframes which can bring them sufficiently into alignment that they can be transferred from a transport or storage cassette to a testing and cutting apparatus. It is a further object of the present invention to provide an improved means of holding a sawframe in place during handling which obviates the above disadvantages.